Distinct features of most Gram positive bacteria are the presence of ipoteichoic acid and a membrane system consisting of a plasma membrane and mesosomes. The purpose of this project is to relate membrane structure with functiion using Staphylococcus aureus as the model test system. The major emphasis is to define the function (or functions) of mesosomes which contain all of the lipoteichoic acid present in the cell. To corroborate the chemical localization of lipoteichoic acid in mesosomes by immunochemical methods, an antisera specific for the polyglycerol phosphate portion of the molecule was prepared. This antisera failed to react with ribitol teichoic acid and gave one precipitin band with deacylated lipoteichoic acid and cardiolipin. Passive hemagglutination inhibition tests showed the exclusive localization of lipoteichoic acid in mesosomes; verifying our previous chemical findings. Also, preliminary data on a radioimmunoassay procedure for the quantitation of lipoteichoic acid is presented.